Convert ReadonlyRootFS to a RunOption
Convert ReadonlyRootFS to a RunOption

This seems to have missed out on an update/refactor at some point and was not
usable in its previous form without duplicating the `runOptionFunc`
scaffolding.

Signed-off-by: Ian Campbell <a8db091655a7f47ecb9c39a2d278e9662e440dc0@docker.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc